The present invention relates to the art of injection molding. The invention finds particular application in accumulator heads for forming tubular thermoplastic parisons, and will be described with particular reference thereto.
Heretofore, various apparatus have been developed for producing tubular, plastic articles. Commonly, pellets have been plasticized by being heated and subjected to pressure by an auger or the like. The flow of plasticized material was forced through passages in a head or die which conformed the plastic flow to the desired pattern, particularly, a tubular shape. Usually, the plastic flow was introduced into the head from the side. The plastic flow split, and the halves flowed around a central core or mandrel meeting on the opposite side thereof. Commonly, the plastic failed to flow back together at the opposite side of the mandrel and form a homogenous junction. Rather, the interface where the flow portions met caused an inherent weakness in the formed tubular product. One way of compensating for this weakness was to divide the plastic flow from the plasticizer in half, and introduce the flows into opposite sides of two concentric tubular head passages such that the two flow junctions were generally opposite. See, for example, U.S. Pat. No. 3,801,254, issued April, 1974 to Albert Godtner.
The injection molding head or die normally included a cylindrical, outer housing defining a cavity therein and a mandrel disposed centrally within the cavity. The concentric tubular passages for conforming the plastic flow to a tubular cross section were defined such that one was within the outer housing and the other was in the central mandrel. The concentric passages discharged the layered plastic flows into an annular reservoir defined between the housing and the mandrel. A reciprocating annular piston or ram was disposed in the annular reservoir for cyclically pumping the concentric plastic layers accumulated therein through an outlet die or into a mold cavity. See, for example, U.S. Pat. No. 4,120,633, issued October, 1978, to Harald Feuerherm.
Problems have been encountered by forming the plastic flow passages through the mandrel and the outer housing. The plastic entrance had to be at one end of the piston or ram and the outlets had to be adjacent the other end in order to provide a passage around the ram into the mandrel. Relatively long flow passages were required in order to traverse this distance, and these long passages, in turn, caused a relatively large pressure drop therein.
The present invention contemplates a new and improved apparatus which overcomes the above-referenced problems and others, and which provides a new accumulator head construction yielding better operational results.